Multichannel television sound (MTS) has been sanctioned by the Federal Communications Commission for use in broadcast television. The sound signal is transmitted on the usual FM audio carrier and includes 1. a left plus right audio signal (L+R) which is modulated on the FM audio carrier so as to be usable by mono television receivers; 2. a left minus right audio signal (L-R) which is modulated on an AM subcarrier superimposed on the FM audio carrier and located at 31.468 Hz from the FM carrier; and 3. a pilot signal having a frequency of 15.734 Hz, the horizontal-scanning frequency, also superimposed on the FM audio carrier.
In order to imrpove the signal-to noise ratio of MTS, the audio signals are spectral companded, i.e., preemphasized in accordance with a prescribed formula, before being broadcast. When received, the audio signals are then correspondingly expanded, i.e. deemphasized, in accordance with corollary of the same formula.
The spectral expander circuits in stereo TV transmitters and receivers employ active filtering, level detection, and variable gain elements which process the audio signals according to the fixed mathematical formula. The expander circuits in a stero television receiver thus require frequency alignment and component adjustment in order to match the compander circuits at the transmitter. A standard, continuous spectrum MTS signal could be used when tuning and adjusting the expander and/or compander circuits so that they precisely match the FCC authorized companding formula, but the cost of such a signal source would be very high as compared to the more common television test signal generators, particularly when used in aligning and adjusting the deemphasis circuits of television receivers.